This invention relates to hydraulic pumps used in a hydraulic system, and more particularly it is concerned with improvement in an internal gear pump or motor (hereinafter referred to as an internal gear pump motor) comprising a housing, a pinion and an internal gear rotatably supported in the housing and meshing with each other, and filler piece means located in a space defined between the pinion and the internal gear for dividing the space into two regions which provide a low pressure region and a high pressure region during operation.
In one type of internal gear pump for developing high pressures known in the art capable of developing a high pressure of over 100 kg f/cm.sup.2 that is commercially available, a pinion and an internal gear are adapted to make backlash-free meshing engagement with each other and a radial piston hydraulically urges the internal gear against the pinion to perform a radial pressure loading function. This type of internal gear pump has succeeded in developing a high pressure of over 300 kg f/cm.sup.2. However, some disadvantages are associated with this type of internal gear pump. The need to maintain the gears in backlash-free meshing engagement entails the use of gears of high precision finishes which are expensive. In addition to the gears of high quality, the radial piston and other parts, such as rubber seal means, of high precision finishes should be used in large numbers, thereby increasing cost. The use of the rubber seal for the radial piston has raised the problem of the service life of the pump being reduced. This type of internal gear pump has not yet been put to practical use as an internal gear motor.
In another type of internal gear pump for developing high pressures known in the art that can reversibly serve as an internal gear motor, the filler piece is formed with one or a plurality of circumferential slits and the pressure oil in the high pressure region is introduced into the slits to provide a seal to the tooth crests of the pinion and the internal gear, as disclosed in Japanese Patent Application Laid-Open number 99,408/77. This construction enables a compact size to be obtained in that the radial piston, rubber seal means and other parts of the internal gear pump described hereinabove can be done without. Also, this construction enables production cost to be reduced because the aforesaid parts can be eliminated and the pinion and the internal gear do not require precision finishes and ordinary finishes can be tolerated in view of the fact that backlash need not be taken into consideration in selecting gears.
However, a split filler piece formed with slits is complex in configuration and requires a large number of parts of small size. Difficulties are encountered in producing such parts, and the filler piece becomes high in cost and short in service life.